Social Media Influencer Marketing (SMIM) is a transformative force in modern marketing. While influencer campaigns have gained popularity globally, empirical understanding of the determinants behind their impact remains underdeveloped. This research investigates key determinants: influencer credibility, follower engagement, and content relevance as primary determinants of campaign success, quantified through engagement rate and sales lift. Further, platform type (Instagram, Facebook, YouTube) and audience age group act as moderators influencing how determinants translate into outcomes [1] impact differently. Utilizing a primary sample of 600 influencer campaigns, we apply structural modelling to uncover robust associations and moderation effects. Findings emphasize influencer credibility and follower engagement as highly influential, with content relevance also important. Platform and demographic moderators reveal nuanced patterns, underscoring the necessity for context-specific strategies in influencer marketing. Theoretical contributions advance understanding of digital marketing dynamics, and practical implications guide marketers optimizing influencer campaigns across diverse social platforms.